Add video_mode FIFO command for CRT video mode switching (#1107)

* Add video_mode FIFO command for CRT video mode switching

- video.cpp: Add video_mode_cmd() that parses custom modelines and
  applies them via video_set_mode(), with hfreq safety check blocking
  modes outside 15625-16500Hz to protect 15kHz CRT monitors
- video.h: Declare video_mode_cmd()
- input.cpp: Dispatch "video_mode" commands from MiSTer_cmd FIFO
  to video_mode_cmd()

* Remove hfreq guard from video_mode_cmd

The horizontal frequency check was limiting the command to 15kHz modes only. Removing it allows custom modelines with any frequency to be applied. Software at the other end must take care of not calling this API with unsafe frequencies.

input.cpp

@@ -5864,6 +5864,7 @@ int input_test(int getchar)
 					cmd[len] = 0;
 					printf("MiSTer_cmd: %s\n", cmd);
 					if (!strncmp(cmd, "fb_cmd", 6)) video_cmd(cmd);
+					else if (!strncmp(cmd, "video_mode ", 11)) video_mode_cmd(cmd + 11);
 					else if (!strncmp(cmd, "load_core ", 10))
 					{
 						if(isXmlName(cmd)) xml_load(cmd + 10);